An extinct group called graptolites are the major fossil
representatives of this phylum (see Fig. 4.31). They are considered
hernichordates because they secreted tubes of protein similar to the modern
hemichordate, Rhabdopleura. Graptolites are often found preserved in black
shale as compressed carbonized impressions, sometimes replaced by pyrite.

Graptolites are thought to have been colonial marine
organisms. The colonies were composed of short tubes connected by a common
canal. There are two main orders of graptolites: (1) Dendroidea, bushlike
graptolites that attached to the ocean floor or to floating seaweed, and (2)
Graptoloidea, colonies consisting of branching stipes from an initial chamber,
which floated or swam weakly in the open ocean.

Graptolites are good index fossils for the lower Paleozoic.
Graptoloids declined and became extinct in the early Devonian. Dendroids became
extinct in Permian times.